Nitrate, nitrite and nitro compounds are often present in non-radioactive aqueous mixed waste streams such as nitrate wastes at metal finishing plants, and chemical and munitions plants. Nuclear materials production facilities also generate waste streams containing both nitrate, nitrite and nitro compounds and radioactive materials. A survey performed in 1981 indicated that there were in excess of 1.2.times.10.sup.8 Kg of nitrate wastes stored at Department of Energy and commercial nuclear facilities in the United States.
In many waste or process streams, the concentration of nitrogen compounds is below 1%, which is insufficient for cost effective removal of nitrates by traditional means. Removal of nitrogen compounds may be complicated by the presence of hazardous chemicals and/or radioactivity. Moreover, nitrate and nitrite compounds at elevated concentration in a waste stream present problems such as nitrogen oxides (NO.sub.x) emission upon disposal by incineration, and public health risk upon disposal by drainage into bodies of water.
Traditional methods for removing nitrogen compounds from aqueous media include ion exchange, extraction, membranes, biological denitrification, and incineration.
The Nalco Water Handbook, 1979, pp. 6-11, describes anion exchange as "(t)he only chemical process that removes nitrate". However, the anion exchange process suffers from a number of disadvantages including 1) the anion exchange resin must be regenerated or disposed of, 2) additional waste is produced upon resin regeneration and/or disposal, and 3) anion exchange resin is damaged by radioactivity.
Extraction and membranes, like anion exchange, generate additional waste.
Conventional bacterial systems usually require a settling pond or biological reactor, are carried out at temperatures below 30.degree. C., require equipment designed to handle great quantities of air and water and require residence times on the order of days to reduce nitrate concentrations to acceptable limits.
In cases where nitrate bearing waste streams are incinerated, undesirable nitrogen oxide (NO.sub.x) emissions which contribute to air pollution and acid rain are produced. NO.sub.x can be combined with ammonia and destroyed by gas phase reactions at temperatures between 1000.degree. C. and 1100.degree. C. (known as thermal deNOx) or by selective catalytic reduction, at temperatures between 650.degree. C. and 750.degree. C. in the presence of a catalyst to convert the NO.sub.x to nitrogen, oxygen, and water. Disadvantages of treating nitrous oxides in the gas phase include, but are not limited to, 1) the large size of the equipment required for handling gases, 2) the high temperature operation, 3) handling potentially corrosive condensate after the gas stream is cooled, 4) additional energy required to evaporate the water, and 5) the cost of disposal of a spent catalyst after processing radioactive wastes.
Research efforts have shown promise, but have neither achieved nitrate removal to drinking water standards, nor achieved substantially complete conversion of nitrates to nitrogen gas. Current national drinking water standards for nitrates are 44 mg/l. Although no standard exists for nitrites, the goal is 3 mg/l.
Research by L. J. Miele and A. J. Johnson, Waste Generation Reduction--Nitrates FY 1983 Status Report, Rockwell International, RFP-3619, Feb. 16, 1984, reports results of nitrate removal methods by use of 1) reducing agents, 2) reaction promoters, and 3) catalysts. However, all but one method provided nitrate removals less than 55%. Only the method using formic acid achieved 96% removal of nitrate. The report concludes that "(n)o aqueous denitrification procedure has produced results equal to . . . thermal methods . . . ".
Further research of denitrification with formic acid is presented in Denitration of Radioactive Liquid Waste edited by L. Cecille and S. Halaszovich 1986. One article, Overview on the Application of Denitration in the Nuclear Field, E. R. Merz, discloses reactions between formic acid and nitric acid resulting in N.sub.2 O, NO.sub.x, carbon dioxide and water. Another article, Chemical Reactions Involved in the Denitration Process with HCOOH and HCHO, L. Cecille and M. Kelm, discloses reactions between nitrous acid and formic acid resulting in ammonia, carbon dioxide, and water. A third article, Denitration of Reprocessing Concentrate by Means of HCOOH, M. Kelm, B. Oser, and Drobnik, discloses reactions between nitric acid and formic acid resulting in N.sub.2 O, NO, N.sub.2, carbon dioxide and water. Overall, between 81% and 99% of the nitrate is converted. However, only 3% of the nitrate is converted to nitrogen gas, with 83% converted to N.sub.2 O and 15-20% converted to NO.
Although it is recognized that formic acid is one of the best reductants of aqueous nitrates, research efforts prior to the present invention have not been successful in obtaining nitrate removal to below drinking water standards and have not achieved substantially complete conversion of the nitrogen in the nitrate to nitrogen gas.
Prior to the present invention, removal of nitrates and nitrites from aqueous waste streams was a difficult and expensive task. The present invention is therefore directed toward a method of removing nitrates and nitrites from aqueous waste streams by converting the nitrogen in the nitrates and nitrites to nitrogen gas with little or no formation of nitrous oxides such as NO, NO.sub.2 and N.sub.2 O. The method of the present invention relies upon aqueous phase reactions at moderate temperatures and pressures without the use of a catalyst and without the subsequent regeneration and/or disposal of a catalyst in both non-radioactive and radioactive waste treatment.